1. FIELD OF THE INVENTION
This invention relates to a universal joint suitable for transmitting torque through a large angle and which, particularly but not exclusively, is suitable for use in the transmission between the power take off shaft of an agricultural tractor and the drive shaft of an agricultural implement. The universal joint of the present invention is of the type comprising a double Hookes joint which comprises a double inner yoke and two outer yokes which are each pivotally connected to the inner yoke by a respective cross member, each cross member comprising a first pair of trunnions in bearings in yoke arms of the outer yoke and a second pair of trunnions in bearings in yoke arms of the inner yoke, and guide means in the inner yoke having bearing means engaging the outer yokes to guide them to provide homokinetic torque transmission.
2. DESCRIPTION OF THE PRIOR ART
A double Hookes joint of the above type is described in DE-A-No. 13 02 735 wherein the guide means comprises a circular disc mounted in the inner yoke for radial sliding movement relative to the axis of rotation thereof. The outer yokes are engaged with the disc by means of a ball headed extension on each outer yoke cooperating with a respective cylindrical bearing guide extending perpendicularly and centrally from each side of the disc. The outer yokes are guided in a plane which bisects the angle formed by the rotational axes of the shafts upon which the yokes are mounted. In a double Hookes joint constructed as described in this patent specification, it is only possible to achieve an articulation of 90 degrees in that plane in which the axes of the cross member trunnion bearings of the two outer yokes are located. It is not possible to achieve an articulation of 90 degrees in the perpendicular plane since the angle is limited by the permissible range of traverse of the outer yokes.
In order to increase the articulation of the joint it has been proposed in GB-A No. 2 012 918 to provide a double Hookes joint wherein the cross members are asymmetric, the distance between the arms of each yoke of the double inner yoke radial to its rotational axis being greater than the distance between the arms of each outer yoke radial to its respective rotational axis. In such a arrangement, it is possible to increase the articulation angle in all planes but, at the same time, the space required for articulation purposes is increased and the diameter of the inner yoke is increased. Such a greater diameter of the inner yoke results in an increase in weight of the rotating mass and, generally, a welded connection is provided between the two halves of the inner yoke. With such a welded connection, the inner assembly comprising the disc and bearing guides cannot be removed so that, in the event of damage, the entire double inner yoke has to be replaced.
However, the principle disadvantage of this construction is in the increase in weight of the rotating mass since, in a double Hookes joint, the two individual joints thereof themselves rotate irregularly with alternating accelerative and decelerative effects.